Hi Caroline. We won't lie, this will probably take multiple experiments and it may never come out quite right, but we'll give you all the info we have! Guar gum is a substitute for xanthan gum. You can try to use Gluten-Free All-Purpose Flour in place of the flours. Use 1/2 teaspoon guar gum for every cup of gluten-free flour. You might want to hold back some of the liquid (a tablespoon or two) since whole wheat absorbs more liquid than all-purpose does, and you're not using a gluten-free whole-grain replacement. If your friend is also unable to have guar gum, we're so sorry — we don't have another substitute in mind that will structurally support a cake and keep it risen. Best of luck!

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